Existing DC motor drives are used to provide control of DC motors in many different applications. One such application, which could be considered a low-power application, would be the use of a DC motor to operate a simple conveyer belt in a factory. Another application, which could be considered a high-power application, would be the use of a DC motor to operate a rolling mill in a steel works. In applications such as these two, where control is to be exercised over the DC motor, a DC drive is provided. The DC drive contains electronic circuitry in the form of power electronics and control electronics to control the speed and torque of the DC motor.
In order to provide good control of the DC motor, the control electronics of the DC drive measure the high voltages present in the power electronics. This arrangement however has a potential safety risk: an operator of the DC drive who is necessarily working adjacent to the control electronics may be exposed to the high voltages of the power electronics by any connection between the power electronics and the control electronics needed to measure the high voltages. To address this, existing DC motor drives include a high-impedance circuit between the power electronics and the control electronics. This provides a good degree of isolation of the control electronics, and hence of the operator, from the power electronics. This isolation is based on having high impedance in the circuit which will limit any current flowing from the power electronics to the control electronics to a safe level.
An example of such an arrangement is shown in schematic form in FIG. 1. With reference to FIG. 1, a DC motor drive 10 includes a high voltage side and a low voltage side. On the high voltage side, which in this example is at approximately 400V, the drive 10 includes power electronics 20. An input side of the power electronics is connected to receive a three-phase power supply L1, L2, L3 and an additional power supply E1, E3. An output side of the power electronics 20 is connected to the armature winding A1, A2 of a DC motor (not shown) and to the field winding F1, F2 of the DC motor. The power electronics 20 are for controllably applying the three-phase power supply L1, L2, L3 to the armature winding A1, A2, and for controllably applying the line voltages E1, E3 to the field winding F1, F2.
On the low voltage side, which in this example is not greater than 30 volts, the drive 10 includes control electronics 30. The control electronics 30 provides a user interface to an operator of the drive 10 and are operable to control the power electronics 20 and hence the DC motor.
Certain components and connections are provided between the high voltage side and the low voltage side of the DC drive 10. Firstly, a switched mode power supply SMPS is provided to power the control electronics 30 from the high voltage power supply E1, E3. In addition, current sensors 40 are provided that are connected to the control electronics and arranged to sense the current in each of the three phases of the three-phase power supply L1, L2, L3, and also in the field winding F1. Finally, a high-impedance circuit 50 is connected between the power electronics 20 and the control electronics 30 to measure the voltage in the power electronics 20 and to provide an indication of this to the control electronics 30 for use in controlling the power electronics 20.
A person operating the DC motor drive 10 will come into close proximity with the low-voltage side of the drive 10. It is therefore desirable to provide a good degree of isolation between the low-voltage side and the high-voltage side to minimise the risk of the operator becoming inadvertently exposed to high voltages and consequent injury. In the arrangement shown in FIG. 1, a good degree of isolation is provided: the high impedance circuit 50 provides a good degree of isolation between the high voltage side and the low voltage side; and the SMPS and each of the current sensors provides a very high degree of isolation between the two sides.
Nevertheless, it is desirable to improve further the safety of DC motor drives.